Scientists Observe Molecular Motions in Real Time

An international team of researchers led by Brown University has used ultra-high-speed X-ray pulses to image subtle motions of a molecule of N-methyl morpholine.

Vibrational motions of N-methyl morpholine. Image credit: Brown University.

“For many years, chemists have learned about chemical reactions by essentially studying the molecules present before and after a reaction has occurred,” said Brown University’s Dr. Brian Stankus, first author of the study.

“It was impossible to actually watch chemistry as it happens because most molecular transformations happen very quickly. But ultrafast light sources like the one we used in this experiment have enabled us to measure molecular motions in real time, and this is the first time these sorts of subtle effects have been seen with such clarity in an organic molecule of this size.”

For the study, Dr. Stankus and colleagues looked at the molecular motions that occur when N-methyl morpholine is excited by pulses of UV light.

“We basically hit the molecules with UV light, which initiates the response, and then fractions of a second later we take a picture with an X-ray pulse,” Dr. Stankus said.

“We repeat this over and over, with different intervals between the UV pulse and X-ray pulse to create a time-series.”

The X-rays scatter in particular patterns depending on the structure of molecules.

Those patterns are analyzed and used to reconstruct a shape of the molecule as the molecular motions unfold.

The experiment revealed an extremely subtle reaction in which only a single electron becomes excited, causing a distinct pattern of molecular vibrations.

The researchers were able to image both the electron excitation and the atomic vibration in fine detail.

“This study is a true milestone because for the first time, we were able to measure in great clarity the structure of a molecule in an excited state and with time resolution,” said Brown University’s Professor Peter Weber, senior author of the study.

A particularly interesting aspect of the reaction is that it’s coherent — meaning when groups of these molecules interact with light, their atoms vibrate in concert with each other.

“If we can use experiments like this one to study how exactly light can be used to direct the collective motion of billions of molecules, we can design systems that can be coherently controlled,” Dr. Stankus said.

“Put simply: if we understand exactly how light directs molecular motions, we can design new systems and control them to do useful chemistry.”

The study appears in the journal Nature Chemistry.

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Brian Stankus et al. Ultrafast X-ray scattering reveals vibrational coherence following Rydberg excitation. Nature Chemistry, published online July 8, 2019; doi: 10.1038/s41557-019-0291-0